The Hashmi family are hoping to benefit from the technique
A couple have been given permission to have a "saviour sibling" in an attempt to aid their sick daughter.
What does the process entail?
The fertility watchdog, the Human Fertilisation and Embryology Authority said in July 2004 that tissue typing tests can be carried out on embryos to see if they are a match for a seriously ill brother or sister - so that the resulting baby could donate stem cells or bone marrow which the child's body will not reject.
The test involves taking one or two cells from an embryo when it is around three days old for testing.
If there is a match, that embryo will be implanted in the mother's womb so it can develop.
Once a baby is born, stem cells are removed from the umbilical cord and stored for four to six months when the transplant to the sick child can take place.
Does this mean doctors can create a "designer baby"?
It is a term widely used to describe the procedure. But experts say that a more apt term is "saviour sibling".
Professor Alison Murdoch of the British Fertility Society stresses: "We are not taking about engineering a child to have a certain hair colour or aesthetic characteristic.
"This is about families being able to make a decision that their new baby could save the life of its older brother or sister:"
Can any family with a sick child could be helped by this technique ?
No. The HFEA stresses their decision will have limited applications. It says each family which wants to help their sick child in this way will have to apply individually to be allowed to proceed.
And it says doctors must have considered all other treatment options before they consider pre-implantation tissue typing.
Has the HFEA looked at similar cases before?
Yes. In 2002, it considered the case of Charlie Whitaker who has a rare blood condition called Diamond Blackfan Anaemia.
The HFEA refused permission for his family to undergo the procedure because it said there was not enough evidence to support its use at the time.
It said then that it was permissible to test and select embryos to prevent the birth of a baby with a genetic disease, but not to select them in order to help another child - a position it has now altered.
Following the ruling, the Whitakers decided to go to the US for treatment, and in 2004 Michelle Whitaker gave birth to baby Jamie who is a near-perfect match to four-year old Charlie.
But wasn't there a case where a family was allowed to use this procedure - the Zain Hashmi case?
Yes. Four-year-old Zain has thalassaemia, a genetic condition.
Doctors say a transplant of "stem cells" from a donor with the same tissue type will "kick-start" healthy blood cell production, but the family have been unable to find a someone who is a close enough match.
The family was given permission to use the tissue typing procedure by the HFEA last year because any selected embryo could be tested to ensure it was not affected by thalassaemia - as well as helping Zain.